1. Field of the Invention
The invention relates to a device and a method of diagnosing abnormality of an air-fuel ratio sensor that detects the air-fuel ratio of exhaust gas of an internal combustion engine.
2. Description of the Related Art
In an internal combustion engine equipped with an exhaust gas purification system that uses a catalyst, it is indispensable to control the mixing proportion between air and fuel in a mixture that is burned in the internal combustion engine, that is, the air-fuel ratio, in order to effectively remove harmful components of exhaust gas through the use of the catalyst. In order to perform such a control of the air-fuel ratio, an air-fuel ratio sensor that detects the air-fuel ratio on the basis of the concentration of a specific component of exhaust gas is provided on an exhaust passageway of the internal combustion engine, and a feedback control is carried out so as to bring the detected air-fuel ratio closer to a predetermined target air-fuel ratio.
Incidentally, if the air-fuel ratio sensor has an abnormality, such as degradation, a failure, etc., the air-fuel ratio feedback control cannot be accurately executed, so that the exhaust gas emission deteriorates. Therefore, diagnosis of abnormality of the air-fuel ratio sensor has been conventionally conducted. Particularly, in the case of engines mounted in motor vehicles, the detection of abnormality of the air-fuel ratio sensor in a vehicle-mounted state (on board) is required by laws or regulations or the like in many countries in order to prevent the motor vehicles from running while emitting deteriorated exhaust gas.
Japanese Patent Application Publication No. 2005-30358 (JP-A-2005-30358) discloses an abnormality detection device for an air-fuel ratio sensor that cyclically increases and decreases the air-fuel ratio through an open-loop control and that detects an abnormality of the air-fuel ratio sensor on the basis of the length or area of the locus of the air-fuel ratio sensor output that increases and decreases corresponding to the cyclical increases and decreases in the air-fuel ratio. Besides, Japanese Patent Application Publication No. 2004-68602 (JP-A-2004-68602) discloses an air-fuel ratio control device that recursively identifies a plant model representing a detection delay characteristic of the air-fuel ratio sensor, and that sets a control gain in the air-fuel ratio feedback control by using a parameter of the identified plant model. This control device stops the serial identification, when performing the diagnosis of response degradation of the air-fuel ratio sensor during the feedback control.
However, the technology described in Japanese Patent Application Publication No. 2005-30358 (JP-A-2005-30358), while being able to discriminate whether the air-fuel ratio sensor itself is normal or abnormal, is not able to discriminate which one of the characteristics of the air-fuel ratio sensor is normal or abnormal. That is, although the air-fuel ratio sensor has a plurality of characteristics, the technology described in Japanese Patent Application Publication No. 2005-30358 (JP-A-2005-30358) cannot discriminate which of the characteristics is abnormal.
Besides, the technology described in Japanese Patent Application Publication No. 2005-30358 (JP-A-2005-30358) performs an active air-fuel ratio control of forcing the air-fuel ratio to increase and decrease for the abnormality detection of the air-fuel ratio sensor. However, a problem of this technology is that if the active air-fuel ratio control is carried out, deterioration of the exhaust gas emission is inevitable.
On the other hand, the technology described in Japanese Patent Application Publication No. 2004-68602 (JP-A-2004-68602) diagnoses degradation of only the response rate, among the characteristics of the air-fuel ratio sensor. Although the air-fuel ratio sensor has other characteristics as well, the technology cannot discriminate whether or not there is abnormality with regard to the other characteristics. In the first place, the technology described in Japanese Patent Application Publication No. 2004-68602 (JP-A-2004-68602) is a technology that relates to the air-fuel ratio control, and that is not specialized in the abnormality diagnosis of an air-fuel ratio sensor. Hence, the parameters of the recursively identified plant models are used to set control gains in the air-fuel ratio feedback control, and the recursive identification is stopped at the time of the response delay diagnosis about the air-fuel ratio sensor.